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290                      EXTINCT GENERA OF  CAMELS                     CHAP.
there are more points of likeness to the ILama, in P, angustidens
to the CameL In both, the orbits are completely encircled by
bone. The nasals are much shortened. The odontoid process of
the axis is still more concave than in Poebrotherium, but not spout-
like as in existing forms. This fact shows that the spout-like
character of the Camels' odontoid process is not a point of affinity
to other Artiodactyles—in fact the occurrence of the same form of
odontoid process in Perissodactyles is enough proof of this. "We
must come to the conclusion that the form is adaptive in all eases.
If we were not obliged on palaeontological evidence to come to this
conclusion, the structure in question is just one which, would be
fastened upon as evidence of genetic affinity; for it is a resem-
blance in a small though distinctive point of structure having no
obvious relation to utility. The metacarpals and metatarsals
have coalesced to form the cannon bones, though a rudiment of
one metacarpal seems to remain. The genera referred to appear
to be on the direct line of descent of the modern representatives
of the family. But there are other forms which are offshoots of
the main stem. Such are &omocam,elit,s, JUschatia, and Ifolo-
meniscus. The last two are Pliocene and American ; the teeth
are much reduced.
The Pecora are a group which possess so many characters in
common that it is not an easy task further to subdivide them.
In all there are but two functional digits on the feet, and the
metacarpals and metatarsals of these are fused. There are 110
upper incisors, and canines in the upper jaw are not universal,
and generally small Horns are confined to this group of the
Selenodontia,1 The premolar teeth are of a simpler form than
the molars. The stomach has four chambers, of which two
may be regarded as belonging to its cardiac half and two to
the pyloric. The former are, in the first place, a large paunch
or rumen, followed by a smaller reticulum, so called on account
of the network arrangement of the folds of its lining membrane.
Connected with the latter, and constituting the first part of the
pyloiic1 half of the stomach, is the psaltarium or ** manyplies," so
eaUed on. aocovmt of the longitudinal folds, like the leaves of a
1 Uatess JVolocerew (see j>. 2S4) was furnished witli boms.